Synopsis During the float-equipped aircraft's take-off run, the pilot noticed a yaw to the left. The pilot corrected the yaw, but it recurred and worsened. The left float separated from the aircraft, the left wing struck the water, and the aircraft capsized, coming to rest on its left wing tip in about 15 feet of water. The pilot and the six passengers were not injured; they exited the aircraft and swam to shore. The aircraft sustained substantial damage. The Board determined that the left front swivel fitting attachment bolt probably moved out of position because the securing nut was either not installed or came off in service. A series of failures ensued, culminating in the separation of the left float from the aircraft. 1.0 Factual Information 1.1 History of the Flight The Beech 18 seaplane was departing a fishing camp at Bradburn Lake, Manitoba, for its third flight of the day en route to the operator's base at Selkirk. After boarding the passengers and loading the baggage for the flight, the pilot taxied the aircraft to a position near the southwest shore of the lake and started the take-off run on an approximate heading of 015 degrees true. During the take-off run, after the aircraft was on the step, the pilot noticed a yaw to the left. He corrected the yaw with the aircraft's rudders and with differential engine power. Shortly thereafter, at about 60 miles per hour1 (mph)2, the yaw recurred and worsened. The left float separated from the aircraft and the left wing struck the water. The aircraft turned sharply to the left, stopped in the water, and came to rest on its left wing tip in about 15 feet of water. The pilot and the six passengers were not injured; they exited the sinking aircraft and swam to shore. The accident occurred at 1040 central daylight saving time (CDT)3 during daylight hours at latitude 5155'N and longitude 9535'W. 1.2 Injuries to Persons 1.3 Damage to Aircraft The aircraft sustained substantial damage when the left float separated and the aircraft stopped in the water and capsized. 1.4 Other Damage The pilot's and the passengers' baggage was damaged by water when the aircraft capsized. 1.5 Personnel Information The pilot had about 6,000 hours of seaplane flying experience. He was working his seventh season with this operator and was certified and qualified for the flight in accordance with existing regulations. 1.6 Aircraft Information The weight of the aircraft at take-off was about 300 pounds under the certified gross weight of the aircraft, and the centre of gravity was within the prescribed limits. The aircraft was equipped with EDO 56-7850A floats, each of which was attached independently to the aircraft fuselage with five struts; the design does not incorporate a spreader bar4. The aircraft is a low-wing design, and the wing and engine cowlings limit the view of the floats from inside the aircraft in flight, and from some positions on the water during a walkaround. The main entry/exit door is located in the left rear area of the fuselage. The pilot reportedly checked the float struts before departing on the first flight of the day. Neither the aircraft's flight manual nor its operating manual states the maximum amount of down wind component that is acceptable for take-off with the aircraft. 1.7 Meteorological Information The weather observed at 1100 CDT at Little Grand Rapids, Manitoba, 16 miles southwest of the site, was as follows: 4,500 feet scattered clouds, visibility 15 miles, winds 240 degrees true at 13 knots. Witnesses reported that the winds at the time of the occurrence were out of the south at less than 10 knots and not gusty, and the surface of the water had light rippled waves. 1.8 Wreckage and Impact Information Examination of the aircraft after the occurrence revealed that the left float's front and rear vertical struts were still attached to the float, but their top fittings (at the aircraft nacelle) were twisted and showed signs of overload failure. The eyebolt fitting of the diagonal side strut failed at the swivel fitting where it attaches to the float. The rear side strut upper attachment bolt at the fuselage was twisted, and the rear swivel fitting bolt, by which the swivel and the diagonal strut are attached to the rear float mount pad, was broken. The mounting pad on the float showed signs of metal smearing. The front side strut was still attached to the fuselage; the attaching bolt was in place and the fitting showed little damage. The float end of the front side strut incorporates a swivel fitting, which attaches to a mounting pad on the float. Neither the front left swivel fitting nor the corresponding mounting pad showed evidence of damage or distortion; the mounting bolt and nut were not recovered. 1.9 Float Installation The Bristol Float Service Manual specifies that castellated nuts secured with cotter pins are required for the strut mounting bolts. The operator reported that castellated nuts and cotter pins were used in the float installation. The float installation manual specifies 14 bolts with castellated nuts for each float; 10 bolts and nuts for the left float were recovered. Nine of these bolts were installed with fibre self-locking nuts. In order for a self-locking nut to lock securely, the thread of the bolt must pass fully through the end of the nut. It was noted that several of the bolts securing the struts and fittings did not pass fully through the end of the securing nut. Experience has shown that float fitting bolts left in service for longer than two years tend to deteriorate from the effects of wear and corrosion. 1.10 Tests and Research The front side strut swivel fitting and the corresponding float mounting pad, and the rear swivel fitting with the attached end of the diagonal strut eyebolt were submitted to the TSB Engineering Branch for examination. After examination of the front swivel fitting and mounting pad, the Engineering Branch concluded that the attaching bolt probably did not break, but moved out of position, either because the nut was not installed or because it came off during service. The bore of the front float mounting pad showed circumferential markings in a narrow band centred approximately one-half inch from the aft end. These marks appeared to have been made recently, and their form was consistent with the threads of an AN7 bolt, which was the type specified for the missing attachment bolt. The Engineering Branch concluded that the marks were probably made by side loads on the assembly while the bolt was partially withdrawn. The bolt that attaches the rear swivel fitting to the rear mounting pad was found bent and broken by a combination of shear and tensile loading, with no evidence of progressive failure. The swivel fitting was twisted but not broken. The eyebolt attaching the swivel fitting to the diagonal strut was found to be bent and broken and the fracture surfaces were typically 45-degree slant fractures characteristic of tensile overload. Moderate surface corrosion was found on the inner wall of the bolt and on its fracture surfaces. Some of the internal corrosion may have been present before the fracture, but it had not significantly reduced the thickness of the bolt wall or contributed to the fracture. There was no evidence of pre-cracking or progressive failure. 1.11 Inspection Schedule The Beech Maintenance Manual does not have a seaplane section. The Bristol Service Manual for the EDO 56-7850 floats for the Beech 18 provides that the floats are to be removed for inspection every 500 hours or every end of season, whichever comes first. The operator is a Transport Canada Approved Maintenance Organization (AMO). The AMO's Beech 18 inspection program approval specifies that the float struts and attachment are to be checked for cracks and general condition, and attaching bolts for security, every 100 hours. There is no specific requirement in the inspection approval for the struts to be removed for inspection every 500 hours or end of season. Among the conditions attached to the approval are the following: the operator is not absolved from responsibility for ensuring that the aircraft is maintained in an airworthy condition; the operator shall ensure that the aircraft is in compliance with all component life limits and other applicable mandatory requirements; the operator shall evaluate for applicability to the program, all recommendations made by the manufacturer of the aircraft and their installed engines, propellers and appliances, as published in maintenance manuals, recommended schedules service bulletins and other technical documents. Where appropriate, the operator shall initiate amendment action. All amendments shall be approved by the Minister. According to Transport Canada records, the operator's inspection approval was not amended to incorporate the Bristol inspection provisions. Some Transport Canada inspectors recommend that operators of the accident aircraft type remove the floats every two years to inspect the struts. According to the maintenance records for the aircraft, the floats and attachment struts were last removed for inspection in April 1988. Since that time, the aircraft's float attachment fittings have reportedly been inspected annually and attachment bolts replaced on condition. All of the float attachment bolts recovered and examined showed little evidence of wear or corrosion. The aircraft had flown about 1,160 hours between April 1988 and the time of the accident. 1.12 Survival Aspects Before commencing the take-off, the pilot completed a passenger briefing, in which he mentioned, among other items, the location of the life-jackets mounted on the fuselage of the aircraft above the passenger seats and the locations of the aircraft exits. The passengers were not required to don the life jackets before take-off. After the float separated from the aircraft and while the aircraft was settling in the water, the pilot exited via the overhead hatch and attempted unsuccessfully to open the main cabin door at the rear of the fuselage. The passengers exited the cabin through the overhead hatch and stood on the wing of the sinking aircraft as the pilot re-entered the aircraft, transmitted a distress call from the aircraft's very high frequency (VHF) radio, and retrieved five life-jackets from their positions on the fuselage adjacent to the passenger seats. The pilot and four of the passengers donned the life-jackets and swam to the nearest shore, a distance of about 500 feet. Two of the passengers swam to shore without life-jackets. The pilot and one of the passengers walked and swam back to the fishing camp, returned with a boat, and took the party back to the camp.